One operation frequently employed in the production of semiconductors is an etching operation. In an etching operation, one or more materials are partly or wholly removed from a partially fabricated integrated circuit. Plasma etching is often used, especially where the geometries involved are small, high aspect ratios are used, or precise pattern transfer is needed.
With the move from planar to 3D transistor structures (e.g., FinFET gate structures for logic devices) and advanced memory structures such as Magnetoresistive Random Access Memory (MRAM) and Resistive Random Access Memory (ReRAM), plasma etching processes need to be increasingly precise and uniform in order to produce quality products. One problem with conventional etching techniques is that etching byproducts, instead of being swept away, are sometimes re-deposited on surfaces where such deposition is not desired. For example, the byproducts may deposit back on the substrate, especially on the sidewalls of features. The byproducts are often metallic or metal rich films. This redeposition phenomenon, and the related problem of incomplete or non-vertical sidewall etching, is particularly problematic when etching non-volatile substances, which is often the case when fabricating advanced devices such as MRAM, ReRAM, cross point memory devices, etc. These etching processes may be dominated by sputtering from ions, a method which typically results in some amount of redeposition on the feature sidewalls.
Unwanted etching byproduct deposition on the substrate can cause many problems including poor etch results and sub-standard devices. For example, the deposition may result in a non-vertical etching profile or other etching non-uniformities. Further, the deposition may cause electrical problems, especially where the structure being etched has interleaving layers dielectric and conductive films. In some cases, unwanted metallic material may deposit on the sidewall of an etched stack, thereby forming an electrical connection between layers that are supposed to be electrically insulated. This connection creates a short circuit in the stack and may result in failure of the device. One example of this type of failure is a short that forms across a magnetic tunnel junction (MTJ) barrier of an MRAM device.
Thus, there exists a need for improved semiconductor fabrication methods and apparatus which permit the removal of unwanted material from a semiconductor substrate (especially material on a side wall of an etched feature) after the substrate has been etched.